The term “biomass” generally refers to the total amount of organisms that inhabit or organic matter that exists in a given area. Particularly regarding plants, plant biomass refers to the dry weight of the plants that exists in a given area. The unit of such biomass is quantified using mass or energy level. The expression “biomass is a synonym of a term “an amount of an organism.” In the case of plant biomass, the term “standing crop” is also used. Plant biomass is generated by fixing carbon dioxide in the air using solar energy, so that it can be captured as so-called carbon neutral energy. Therefore, an increase in such plant biomass has effects of terrestrial environmental protection, prevention of global warming, and reduction of greenhouse gas emissions. Hence, technologies for increasing plant biomass have high industrial importance.
In addition, plants are cultivated for their partial tissues (e.g., seeds, roots, and leaf stems) or for production of various substances such as fats and oils. For example, as fats and oils produced by plants, soybean oil, sesame oil, olive oil, coconut oil, rice oil, cottonseed oil, sunflower oil, corn oil, safflower oil, palm oil, rapeseed oil, and the like are conventionally known and broadly used for household or industrial applications. Also, fats and oils produced by plants are used as raw materials for biodiesel fuel or bioplastics, allowing the applicability thereof to spread as alternatives to petroleum as energy sources.
Under such circumstances, improvement of productivity per unit of cultivated area is required for industrially successful fat and oil production using plants. Assuming that the number of cultivated plants per unit of cultivated area remains constant, it is understood that improvement in fat and oil production per individual plant is needed. When fats and oils are collected from seeds harvested from plant bodies, it is expected that improved fat and oil production per individual plant can be achieved by a technology for improving the seed yield per individual plant, a technology for improving the fat and oil contents in seeds, or the like.
Technologies for increasing the fat and oil production from plant seeds are mainly divided into those based on improved cultivation techniques and those based on development of cultivars for increased fat and oil production. Methods for developing cultivars with increased fat and oil production are mainly divided into conventional breeding techniques mainly composed of mating technologies and molecular breeding methods using genetic recombination. As technologies for increased fat and oil production using genetic recombination, A) a technology that involves altering the synthesis system for seed triacylglycerol (TAG), which is a major ingredient of plant fats and oils, and B) a technology that involves altering various control genes for controlling plant morphological formation, metabolism, and the expression of genes involved therein are known.
Possible examples of method A) above include methods for increasing the amount of TAG synthesized using sugar produced by photosynthesis as a raw material. These include (1) a method that involves enhancing activity for the synthesis of fatty acid or glycerol, which is a component of TAG from sugar; and (2) a method for enhancing the reaction by which TAG is synthesized from glycerol and fatty acid. Concerning such methods, the following technologies have been reported as technologies using genetic engineering techniques. An example of (1) is provided in a report (Plant Physiology (1997) Vol. 11, pp. 75-81) wherein it was noted that seed fat and oil contents were improved by 5% via overexpression of cytoplasmic acetyl-coenzyme A carboxylase (ACCase) of Arabidopsis thaliana in rapeseed plastids. Also, an example of (2) is provided in a report (Plant Physiology (2001), Vol. 126, pp. 861-874) concerning a technology for increased fat and oil production via overexpression of DGAT (diacylglycerol acyltransferase), which undergoes acyl transfer to the sn-3 position of diacylglycerol. In the report regarding this method, fat and oil contents and seed weights were increased as the DGAT expression levels were increased, so that the number of seeds per individual plant could increase. Arabidopsis thaliana seed fat and oil content was increased by 46% with the use of this method, and the fat and oil content per individual plant was increased by approximately 125% at maximum.
In addition, a possible example of method B) above is a method that involves controlling the expression of a transcriptional factor gene involved in control of the expression of a biosynthesis system enzyme gene. An example thereof is given in WO01/36597. In WO01/36597, a technique was employed that involves producing recombinant plants through exhaustive overexpression or knock-out of a transcriptional factor and then selecting a gene that enhances seed fat and oil contents. WO01/36597 states that seed fat and oil contents were increased by 23% through overexpression of the ERF subfamily B-4 transcriptional factor gene. However, WO01/36597 does not state increases or decreases in the fat and oil content per individual plant. Plant J. (2004) 40, 575-585 describes that seed fat and oil contents can be improved by overexpression of WRINKLED1, the transcriptional factor containing the AP2/EREB domain.
Furthermore, when a hydrocarbon component such as cellulose contained in plant bodies is glycosylated and then alcohol is produced by fermentation, fat and oil components contained in plants become impurities that can cause reduced glycosylation efficiency in a glycosylation step. Therefore, if fat and oil contents can be decreased, glycosylation efficiency in a glycosylation step can be improved and thus improved alcohol productivity can be expected. For example, Plant J. (2004) 40, 575-585 discloses that in the case of the WRI1/ASML1 (AP2 family transcriptional factor: AGI-code; AT3g54320)-deficient line, seeds were wrinkled and the fat and oil contents were decreased. Furthermore, WO01/35727 discloses the following: the seed fat and oil content was decreased by 13% through overexpression of AT3g23250 (MYB15); the seed fat and oil content was decreased by 12% through overexpression of AT1g04550 (IAA12); and the seed fat and oil content was decreased by 16% through overexpression of AT1g66390 (MYB90).
Moreover, several attempts to improve biomass have been carried out. For example, Proc. Natl. Acad. Sci. U.S.A., 2000, Jan. 18; 97(2), 942-947 discloses that plant organ cell number, organ size, and individual plant size were increased through overexpression of the At4g37750 (AINTEGUMENTA) gene. Similarly, Plant Cell, 2003, Sep.; 15(9), 1951-1961 discloses that when overexpression of At2g44080 (ARL) was caused, plant organ cell number, organ size, and individual plant size were increased. Also, Plant J. (2006) July, 47(1), 1-9 discloses that cell division was activated through overexpression of At1g15690 (AVP1), so that individual plant size was increased. Furthermore, Development 2006, January; 133 (2), 251-261 reports that when At5g62000 (ARF2) was deficient, seeds and flower organs became larger in size.
However, although the above molecular breeding methods for improvement of various characters have been developed, no technology has reached a practical level that would allow both increased biomass and improved or decreased fat and oil productivity.
This may be because truly excellent genes remain undiscovered and because novel recombinant cultivars effective at test stages are unable to exert effects as desired at practical stages under various natural environments. Furthermore, regarding quantitative character such as increased plant weight and productivity of a target substance, many genes are involved in various steps, ranging from control systems to metabolic systems. Hence, it has been difficult to discover and develop a truly excellent useful gene for improvement of quantitative characters. Objects required to address these problems are: discovery of a novel gene with drastically high effects; and development of a gene capable of exerting effects under practical environmental conditions, even if its effect levels are equivalent to those of conventional genes. Furthermore, it is expected that practical levels would be achieved by the simultaneous use of a plural number of genes, even if each of the genes has effect level equivalent to or lower than those of conventional genes. Accordingly, another object is to develop a plurality of genes having different functions.